Electric vacuum apparatuses which relate to vacuum arc extinction chambers and vacuum gaps can operate reliably only when their contacts or electrodes are subject to special preparation treatment. Adequate methods of contact and electrode preparation provide for higher operational reliability and greater electric strength of electric vacuum apparatuses.
Known in the art are contact and electrode preparation methods which consist in the degassing of contacts and electrodes by initiating a d.c. discharge between them in the environment of an inert gas or hydrogen (cf. the USSR Inventor's Certificate No.588,573, Int.cl. HO1H 33/66, published in bulletin "Discoveries, Inventions, Industrial Designs and Trademarks", No. 2, l978; Japanese Application No. 50-39,827, Int.cl. HO1H 33/66).
Such degassing methods make it possible to remove surface impurities and gas contaminants without changing the structure of the contact or electrode, a feature not providing for an improvement of the electric strength and operational reliability of an electric vacuum apparatus.
There are methods of contact preparation in which contacts are subject to heat treatment in an electric furnace, in the environment of hydrogen or in a vacuum (cf. Japanese Application No. 51-3073, Int.cl. HO1H 33/66) and also methods utilizing electron beam for the purpose (cf. Japanese Application No. 51-36.468, Int.cl. HO1H 33/66).
With the above methods, a contact is so prepared that its surface layer contains higher amount of a fusible component (copper) than the remaining contact areas. These methods, while providing for higher electric strength of an electric vacuum apparatus, result in a considerable increase in contact resistance, which imposes certain limitations on the rating of the current passing through a vacuum arc extinction chamber. Moreover, these methods can be implemented only in the case when there exists a large difference between the boiling points of the different contact metals as in the case of tungsten-copper material.
There is a contact preparation method dealing with an improvement of the electric strength of a vacuum arc extinction chamber by initiating an arc and cutting out the current that exceeds its rated value (cf. Japanese Application No. 51-8176, Int.cl. HO1H 33/66).
This method is capable of eliminating merely surface contact defects, which cannot provide for a high electric strength and, as a consequence, for a high working voltage of the chamber.
Known in the art is a method of preparing contacts and electrodes of electric vacuum apparatuses by using a heavy-current short-duration arc (cf. the USSR Inventor's Certificate No. 756,510, Int.cl. HO1H 33/68, published in bulletin "Discoveries, Inventions, Industrial Designs and Trademarks", No. 30, l980). According to the method, an arc is stricken between the electrodes or separated contacts of an electric vacuum apparatus by applying a current pulse of a length of 5 to 20 ms at 10 to 100 kA, with the space between the electrodes or contacts being maintained within the range 5 to 100 mm. Under these circumstances, a concentrated thermal flux acts on the surface of the electrodes or contacts for 0.5 to 20 ms and that surface is then subject to cooling.
As a result, the surface is degassed and chemically sorbed gases are removed from it at a minimum contact (electrode) erosion. This means that the surface layer of the contacts (electrodes) neither melts nor changes its structure. Thus, the method fails to improve efficiently the electric strength and operational reliability of electric vacuum apparatuses.